Japan’s crown Princess Masako has said she feels “insecure” about her duties as empress when her husband, crown prince Naruhito, ascends the Chrysanthemum throne next year, as her doctors warned that she continued to experience symptoms of her stress-related illness.

In an unusually candid statement to mark her 55th birthday on Sunday, Masako added that she would do her best after Naruhito succeeds his 84-year-old father, Akihito, who on 30 April will become the first Japanese emperor to abdicate for 200 years.

“Even though I feel insecure about how helpful I will be when I think about the days ahead ... I want to devote myself to the happiness of the people so I will make an effort to that end,” Masako said.

Speculation is mounting over how she will approach her new role given her much publicised 15-year battle with an illness her doctors have described as an adjustment disorder.

In 2004, Naruhito suggested the pressures of imperial life had impacted negatively on his wife, a former diplomat. “It is true that there were moves to negate Masako’s career and her personality, which was influenced by that career,” he said in a rare attack on the straitjacket culture of the imperial household.

Masako, who was educated at Oxford and Harvard universities, also came under pressure to produce a royal heir amid concern over the lack of males in the imperial family.

The couple, who married in 1993, have a 17-year-old daughter, princess Aiko, but she is forbidden from becoming empress by Japan’s male-only succession law.

Masako added, however, that improvements in her health had enabled her to carry out more official duties this year, including her first full appearance at a biannual imperial garden party for 15 years.

“I am delighted at the fact that I can perform more duties than before as I have tried to improve my physical condition,” she said. “I will continue to work for my recovery and strive to do my best in performing as many official duties as possible.”

In a separate statement, Masako’s doctors said more frequent public appearances had boosted her confidence, but added that they occasionally left her feeling tired.

In a rare televised address in August 2016, Akihito said he feared his age and declining health would leave him unable to perform his duties as emperor. He has had heart surgery and treatment for prostate cancer.